1. Field of the Invention
The present invention relates to a technique for switching between antennas for a reader/writer in an RFID (radio frequency identification) system.
2. Description of the Related Art
An ID tag used for a noncontact tag in an RFID system comprises internally a transmission/reception element. The ID tag transmits and receives radio waves or electromagnetic waves to and from a reader/writer by using the transmission/reception element to exchange a series of information stored by the ID tag. As for a power supply circuit, there is a known technique comprising an antenna coil which resonates with a received radio frequency of a reader/writer when coming into proximity thereto to extract power by rectifying and smoothing the output from the antenna coil (e.g., patent document 1).
A reader/writer comprises an antenna for transmitting and receiving radio waves or electromagnetic waves to and from an ID tag. Here, the intensity of radio wave which the ID tag received from the reader writer varies with position and/or direction of the ID tag relative to the reader/writer. Accordingly there is a practical reader/writer which comprises a plurality of antennas in an attempt to improve reception sensitivity for an ID tag regardless of positional relationship between the reader/writer and the ID tag (e.g., refer to patent documents 2 and 3). Such a reader/writer is capable of positioning antennas in respectively different directions, such as two or four directions. Such a configuration enables the ID tag to exchange data with a reader/writer by way of any one of the antennas, thereby improving the quality of communication between the ID tag and reader/writer as compared to the case of only-using one antenna.
[Patent document 1] Japanese laid-open patent application publication No. 2004-303175 (Abstract, FIG. 1 and paragraph 0006)
[Patent document 2] Japanese laid-open patent application publication No. 2004-227315 (Abstract, FIG. 1 and paragraph 0008)
[Patent document 3] Japanese laid-open patent application publication No. 2004-328605 (Abstract, FIG. 1 and paragraph 0008)
Incidentally, if a plurality of antennas is equipped for diversity as described above, there is a possibility of radio wave interference occurring in a specific direction. Communication between the ID tag and reader/writer could possibly be disrupted at a point where the interference occurs even though it is within the range of radio communication in terms of distance. In this case, the problem will be solved if an additional antenna can be installed for covering the direction of interference occurrence. An additional antenna cannot be installed, however, if the reader/writer does not have a plurality of antenna ports or connecting leads, or all the equipped antenna ports or connecting leads are already in use. Besides, if the reader/writer per se is to be changed to increase the number of antenna ports, further work and capital investment will be required.